1. Field of the Invention
The present invention relates to a radiation transmission type target, a transmission type radiation tube using the target, and a radiation imaging apparatus using the radiation tube.
2. Description of the Related Art
A transmission type X-ray tube is a vacuum tube which includes an anode, a cathode and an insulating tube, accelerates electrons to be emitted from an electron emission source of the cathode with a high voltage applied between the anode and the cathode, and irradiates a metal target provided on the anode with the accelerated electrons to make the metal target generate X-rays. The transmission type X-ray tube is adopted in an X-ray generating apparatus for medical use or industrial use.
Such a general transmission type X-ray tube accelerates and converges an electron beam which has been emitted from a filament of the cathode, and makes the accelerated and converged electron beam collide with the target metal provided on a substrate in an X-ray transmission window on the anode side to make the target metal generate X-rays in a transmitting direction. In the structure of the transmission type X-ray target which generates the X-rays, an insulator of a ceramic or glass with high heat-resistance is used for the substrate in the X-ray transmission window, which has the target metal vapor-deposited on the inner surface. In addition, as a material of the target metal, a metal, which is a high melting point metal and is a heavy metal, is selected. In addition, in such an X-ray tube, the electron beam which has been accelerated by the high voltage collides with the target, the periphery of an insulating portion is electrostatically charged with the electrons and reflected electrons, which are eventually discharged to destroy the target structure and lower the withstand voltage, and consequently a stable operation has been impaired.
As for a countermeasure for preventing the electrostatic charge in order to solve these various problems caused by the influence of the electrostatic charge, a target structure having an antistatic film imparted therein is described in Japanese Patent Application Laid-Open No. 2002-352754. It is specifically disclosed that an X-ray target of a transmission type employs an insulator such as a ceramic or glass for a substrate in the X-ray transmission window, has an antistatic film made from a metal other than the target metal formed on the inner surface thereof, and has the target metal vapor-deposited thereon. Japanese Patent Application Laid-Open No. 2002-352754 describes that the X-ray target can stably operate because of having the antistatic film provided therein so that the periphery of the insulator is not charged with the electrons which have collided with the target.
However, a conventional X-ray tube has the following problems in the prevention of the electrostatic charge. In other words, the antistatic film can prevent the inner surface of the substrate in the X-ray transmission window from being electrostatically charged, because the substrate in the X-ray transmission window has the antistatic film made from the metal other than the target metal formed on its inner surface, and the target metal is vapor-deposited on the antistatic film. However, on the outer surface of the substrate, in other words, on a surface opposite to a surface on which the target material in the X-ray transmission window is formed, the insulating surface of the ceramic or the glass is exposed which is the substrate in the X-ray transmission window. Because of this, electrons which have collided with the target substrate or positive ions which have been ionized due to the emitted electrons are deposited on the target substrate, and cause an electrostatic charge. As a result, the conventional X-ray tube has had such problems that a rise in an electric potential due to the electrostatic charge causes electric discharge, and the electrostatic charge of the target substrate hinders a stable operation.
An object of the present invention is to provide a radiation tube which solves the above described problems, achieves the prevention of the electrostatic charge and can stably operate.